1. Field of the Invention
The present invention relates to compact lens systems.
2. Description of the Prior Art
Compactness in a lens system is measured by the small sizes of clear aperture diameters of the lens elements and by the shortness of the front vertex to rear vertex distance. Compactness is influenced by many requirements such as the F-number, the field of view (or the field angle), focal length and relative illumination or vignetting.
The specific application for which the lens system is designed will dictate such factors as the number of the lens elements, the lens system's complexity, and the required image quality. These factors also influence the lens system's compactness. For example, it is fairly easy to design a compact 1:1 re-imager, symmetrical about an internal aperture stop. It is also fairly easy to design a compact lens system that has no appreciable field of view. It is much harder to design a non-symmetrical compact lens system for an application requiring an excellent image quality over a large field of view (i.e., a field of view of over approximately 10 degrees).
When a lens designer is required to design a non-symmetrical compact lens system with a large field of view, the designer will usually first come up with a starting point lens system design that substantially satisfies the above mentioned requirements. Then the designer will optimize (via the use of a commercial lens design program) this starting point lens design for small clear aperture diameters by targeting the heights of the upper and lower rim field rays originating from the edge of the field of view. More specifically, a lower rim field ray is used to control the front aperture diameter of the lens system and the upper rim ray is used to control the rear aperture diameter.
This method is inefficient and results in small clear aperture diameters only through trial and error. Thus there exists a need for an efficient method that results in compact lens system designs.
Non-symmetrical compact lens systems with large fields of view are useful in many applications. For example, many camera lens systems require compactness. Relatively compact camera lens systems are disclosed, for example, in U.S. Pat. Nos. 5,243,468; 4,521,084; 4,568,151; 4,576,448; and 4,606,607. However, although these lens systems have small front-vertex-to film plane distances, the lens elements' clear aperture diameters are quite large.
A cluster lens printer is a printer that simultaneously uses a plurality of lens systems to produce a plurality of images. These lens systems share a common object and a common image plane. The clear aperture diameters of such lens systems need to be small so that the individual lens systems of the cluster lens system can fit next to each other. Because of this requirement, a cluster lens printer cannot utilize lens systems such as the one disclosed in the above-mentioned patents. Other applications may also require a compact lens system with small clear aperture diameters.
Some lens systems, such as lens systems for camera objectives achieve compactness by reducing clear aperture diameters by vignetting, i.e. by cutting the field rays off. This vignetting causes the resultant images to look bright at the center and dark at the corners. Although a relatively large amount of vignetting (.apprxeq.40% to -50%) can be tolerated in a camera lens, other applications cannot tolerate that much vignetting. For example, a large variation in illumination (due to vignetting) at the image is not acceptable output in a commercial quality printer. For good quality image production, only a minimum amount of vignetting (i.e., 15% and preferably much less) can be tolerated in a printer lens system.
Thus, there exists a need for compact lens systems having both a small front-to-rear vertex distance, small clear aperture diameters, and little or no vignetting.